The present invention relates to a device for applying pulsed radial stresses to the wall of a well and, more particularly to an emitting device adapted to be lowered to a certain depth in a well drilled in the ground for generating seismic pulses.
A source of pulses for wells has a number of advantages. Using such a source, seismic prospection operations may be carried out from well to well which allow a high resolution power to be obtained or else oblique seismic profiles to be rapidly plotted by picking up the emitted waves by means of a plurality of receivers disposed on the surface.
It is moreoever more efficient than a source disposed on the surface. An appreciable part of the acoustic energy produced by this latter is in effect emitted under the form of unusable surface waves or Rayleigh waves and the part of this energy which is emitted in the form of P or S type waves is considerably attenuated by the surface layer or weathered zone. The P or S wave energy yield of a well source emitting under the layer is therefore much higher.
Among conventional well sources may be mentioned those which use solid explosives or explosive gas mixtures of the type proposed in, for example, U.S. Pat. Nos. 3,408,122 and 3,858,167. Such sources produce very powerful seismic pulses but the frequency spectrum of these pulses comprises a considerable portion of high frequencies without practical interest in the seismic prospection field. Moreover, the power of the explosions generally results in damaging the well or in impairing the neighboring regions, so that subsequent repetition of seismic firing at the same positions is difficult.
Seismic pulses may also be generated by air gun type sources of the type proposed in, for example, French Pat. No. 2,311,322, wherein sudden discharges of compressed air are provided inside the well filled with water. Such sources produce powerful acoustic waves but require, for their operation, the use of a compressor or compressed fluid reservoirs which, because of the limited space in the drilled hold, must be installed at the surface and connected to the source through piping.
Another conventional source is proposed in, for example, U.S. Pat. No. 4,207,961, which source is adapted for generating acoustic waves in a well containing water by the sudden ejection of liquid in a direction perpendicular to the axis of the well combined with suction in the opposite radial direction, with these sudden movements of liquid being caused by the movement of a mobile assembly under the action of electromagnetic forces. This proposed source is used, in particular, for emitting S-type waves in a predominate radiation direction parallel to the axis of the well.
In accordance with the invention device is provided which allows very powerful radial stresses to be transmitted to formations surrounding a well. The device comprises a body adapted to be lowered in to a well at the end of the cable provided with electric power supply and signal transmission lines, with the body being connected to at least one anchorage shoe movable between a rest position and an extended position in which the anchorage shoe is applied against the wall of the well, under the action of at least one hydraulic jack actuated by a main hydraulic system disposed in the body and controlled through the cable.
In accordance with further features of the present invention, means are disposed inside the body and also controlled through the cable for applying brief pulsed radial stresses to at least one anchorage shoe, when the anchorage shoe is in abutment against the wall of the well.
Advantageously the means for applying the intermittent radial stresses comprise, for example, an auxiliary hydraulic system communicating intermittently with the control cylinder of the anchorage shoe, so as to discontinuously apply an additional anchorage force to the anchorage shoe.
The device of the invention has a very good energy efficiency to the extent that it operates by application of short pulses to one or preferably more anchorage shoes already anchored against the walls. Its P type wave directional diagram is not very directional when compared with that of seismic sources of the tangential type, which makes it particularly well adapted to the seisimic prospection from well to well or to so called reverse vertical seismic prospection in which the emitted waves are picked up by an assembly of receivers disposed on the surface.
Furthermore, since the device is contained in a body which is connected to a surface installation through a single cable providing suspension, electric supply and control thereof, it is adapted for operating over a wide range of depths.